The present invention relates to a system for regenerating a synchronized carrier wave for demodulation use at a receiving station, and more particularly, to a system for regenerating a carrier wave to be used for demodulation in a vestigial sideband (VSB) transmission system of an amplitude over-modulation type.
Generally, in the transmission of television video signals and the like through a coaxial cable, it is the usual practice to employ amplitude modulation requiring the narrowest sideband distribution. In this case, the degree of modulation in the amplitude modulation is set at a value higher than 100% in order to effectively utilize the load capacity of the transmission line. Furthermore, since the video signal contains very low frequency components, it is difficult to employ a single sideband (SSB) transmission system as is the case with the transmission of a telephone signal, and therefore, a vestigal sideband transmission system is employed.
Hereto, a variety of the so-called over-modulation systems have been proposed in which the degree of modulation is set at a value higher than 100% as indicated above. In achieving the over-modulation, a doublebalanced modulator is generally used. In a typical example of the over-modulation system, the balancing of a modulator with respect to a carrier wave is adjusted during a modulation process by means of a D.C. bias circuit and the excess carrier ratio (ECR) is preselected according to the transmission conditions so that ECR= 0.5 .about. 0.65. Another example of the over modulation system is a carrier-suppression multi-channel television signal transmission system in which no D.C. biasing is provided for the modulator, the adjustment of the modulator balancing being achieved similarly.
At the receiving station, it is necessary to obtain a distortionless reproduction of the original modulating signal from the transmitted signals which have been over-modulated as described above at the transmitting station. To this end, it is important that a carrier wave generated for demodulation at the receiving end be synchronized both in frequency and in phase with the carrier of the modulated wave. The so-called homodyne detection system is usually employed for this purpose, in which the detection with modulation product is carried out by making use of this demodulation carrier wave. In this system, it is an essential condition to synchronize the demodulation carrier wave with the carrier wave contained in said modulated wave. In order to synchronize the demodulation carrier wave with the modulated carrier wave, a system for regenerating a carrier wave by synchronized sampling is employed. This system is based on the fact that when the carrier is subjected to over-modulation it has a phase which reverses depending upon the contents of the modulating television video signals, but the phase of the carrier wave in the portions corresponding to a synchronizing signal is not affected by the contents of the video signals. More particularly, the method employed in the system is first to determine the time positions in the transmitted modulated wave which correspond to the synchronizing signal and thus are not affected by the contents of the video signals, than to produce a carrier wave burst for each synchronizing signal by sampling the modulated carrier wave upon each synchronizing signal, and finally to derive a carrier wave for demodulation from the carrier wave burst by means of a narrow band filter whereby to eliminate a phase difference between the carrier of the modulated wave and the regenerated carrier.
Accordingly, in the aforementioned system for regenerating a carrier wave by synchronized sampling, it is an essential condition to detect, with high stability and reliability the time position of the synchronizing signal in the modulated carrier signal. However, the discrimination of the synchronizing signal is not readily achievable because the relationship between the synchronizing signal and the picture signal depends upon the modulation systems and the contents of the video signals as described above. More particularly, as we have described above, various types of over-modulation systems are available among which the modulated carrier wave signals have distinct and different waveform characteristics. For instance, even in a modulation system having a fixed ECR, the waveforms for ECR.gtoreq.0.5 and for ECR=0.5 are quite difference, and further in case of the DC elimination system, the modulated carrier wave signal shows a waveform different from that available in the fixed ECR system. Therefore, in regenerating the demodulation carrier wave, a regenerating system which effectively makes use of the characteristics of the waveform is employed.
Among the fixed ECR systems, one system is applicable only to the case of ECR&gt;0.5 while another system is equally applicable to the case of ECR=0.5. However, these proposed systems cannot be applied to a system in which the ECR is not fixed, that is, the DC elimination system. On also, there has been proposed a system in which separation of the synchronizing signals is possible in spite of the DC elimination involved. However, in this particular system, there is likely to be a failure to start the proper operation of the system depending upon the picture signals, in which case a proper carrier wave may not be obtained.
It is, therefore, an object of the present invention to provide an improved system for regenerating a demodulation carrier wave with a view to eliminating the above-outlined difficulties.
Another object of the present invention is to provide a system for regenerating a demodulation carrier wave, which can be activated with increased reliability in either the fixed ECR transmission system or the DC elimination transmission system, and which can provide a normal phase carrier wave in spite of the occurrence of the picture pattern variation and/or level variation.